The invention of this application involves an improved design of a thread forming fastener and a unique method of forming threads in a work piece. The thread forming fastener, with which this application is concerned, generates a thread within a pilot hole of a workpiece as the fastener is driven into the hole. The thread in the work piece is cold formed or swaged by the fastener. In the cold forming process, the fastener deforms the material of the work piece and therefore creates a significant amount of friction which must be overcome by the driving force applied to the fastener.
In order to reduce the driving force required in the use of thread forming fasteners, a trilobular design was developed which provides the fastener with a pitch cross section of arcuate triangular shape. Such a cross section includes alternating high and low portions on the crest of the fastener as it progresses along the pitch surface. Initially this shape was formed over the full length of the fastener. This provided a reduction in the required driving torque as thread forming was accomplished through engagement of only the high portions of the fastener thread, thereby sharply reducing the friction caused by the fully engaged circular thread configuration. The reduced thread engagement, however, still existed when the fastener was fully engaged and therefore detracted from the holding power of the fastener. Although fasteners of this type had good thread forming qualities at reasonable driving torque, they lacked the superior holding capability of a standard cylindrical crested thread. There is an inherent conflict of design in this type of fastener between the desired low driving torque of the thread forming and the desired high surface contact or holding power of the cylindrical thread.
An early fastener of a lobular design is shown in the reference Phipard, Jr., U.S. Pat. No. 3,246,556 which describes a fastener having a holding zone 8 of circular cross section at the top of the fastener and a thread forming zone of tapered lobular cross section extending the remaining length of the fastener.
In the patent which issued to Muenchinger, U.S. Pat. No. 3,681,963, a trilobular design is introduced with a cross section having more arcuate sides. The shank of the fastener of the '963 patent is divided into zones along its length, namely a circular zone and a tapered zone. The trilobular shape is confined to the tapered zone and it is this zone that forms the thread in the workpiece while the circular or cylindrical zone provides the full surface contact and holding power of a standard fastener. Typically the thread forming or tapered zone is of short length involving between 2 to 3 pitches of the thread. This means that all of the thread forming work is accomplished in this distance.
A problem with the multi-zone configuration is that it is difficult to obtain fully formed crested threads on the fastener in the transition zone between the trilobular cross section and the circular cross section. This problem is not clearly recognized in either the '556 or '963 patents. The cross section of the fastener in the transition zone does not allow for a symmetrical application of force by the opposing die and this will hinder the formation of fully crested threads on the fastener, even at the apex of the lobes, in the thread forming zone. This is critical as it is the fastener threads in this zone that completes the forming of the threads in the work piece to their full diameter.
The forming of threads on a multi-zoned blank having differing diameters, i.e., in the holding zone and the pilot zone, creates a differential in rotational distance for each zone as threads are formed on the blank in the roll die. The blank in the smaller diameter zone tends to move faster. This creates a force on the blank that will lap material over the crest of the fastener and results in a poorly formed thread. Since, in the case of the trilobular cross section, the width across the blank remains the same, no relief from this problem is provided. It is the purpose of this invention to provide such relief and allow slippage during the rolling process which will allow threads to be formed in the forming and holding zones without detrimental laps on the thread flanks.
Another problem of the multi-zoned fasteners of the prior art is the thread forming work must occur in the relatively short tapered thread section of two to three pitches. This forces all of the work to be accomplished by only six to nine lobes of the fastener, thereby increasing the torque necessary to perform the work and places an even greater burden on the poorly formed threads of this section of the fastener. In practice it has been found that users of these fasteners tend to use a larger pilot hole to overcome the high installation torque otherwise required. This reduces the engagement of the fastener threads in the work piece to a maximum of 70 to 75 percent of the theoretical maximum.
A more recent trilobular fastener design is described in the reference Fulmer, U.S. Pat. No. 5,242,253. This design adds a third alignment zone at the insertion end of the fastener. In addition the lobes of the threads have a heel following the thread forming arc of the lobes which provides increased friction relief. Since the lobes are arranged in a spiral which is transverse to the thread spiral an enhanced holding pattern is touted over the axial aligned lobe of prior configurations because the material of the work piece tends to fill in behind the heel. There is an attempt in the '253 patent to form fully crested threads in the trilobular transition zone by using a blank that is cylindrical over its full length and not tapered. This produces similar problems to the multi-lobed fasteners in that the lobes in the transition zone are poorly formed due to the round body dominating the rolling and allowing pressure to be applied to the changed cross section.
The configuration of the '253 patent will also cause problems in the gauging of the fastener. In measuring the pitch diameter of the fastener, rollers are used which engage the fastener at three locations about the periphery of the fastener. If the gauge rollers cannot contact surfaces which are radially aligned a faulty reading may occur.
It is the purpose of this invention to construct a fastener in which the holding power is maximized while minimizing the frictional effects of the thread forming process and facilitating the gauging of the finished fastener. The thread forming stress on the fastener is carefully balanced with its strength and the thread stripping limits of the workpiece to avoid the common installation problems, while insuring a greater engagement of the fastener threads in the workpiece, thereby increasing the load carrying ability of the engaged fastener. The overall shape of the fastener is designed for ease of manufacture while insuring a fully crested thread throughout the length of the fastener and complete working lobes particularly at the transition section between the circular cross section of the holding zone and the lobular cross section of the thread forming zone.